Electrical musical instrument



Feb. 21, 1950 1.. HAMMOND ETAL 2,493,367

ELECTRICAL MUSICAL INSTRUMENT Filed Sept. 15, 1944 s Shets-Sheet 1 f)? 2/9/220/"5 La awe/z? Ham/22 0/? a L. HAMMOND ETAL ELECTRICAL MUSICAL INSTRUMENT Feb. 21 1950 8 Sheets-Sheet 2 Filed Sept. 15, 1944 770T 'ccwneoL c/xecu/rs fn exzzans la are/2.5 Hammond a John M flame/"2 8 Sheets-Sheet 6 L. HAMMOND ET AL ELECTRICAL MUSICAL INSTRUMENT Filed Sept. 15, 1944 m 2 2 we 0/15 7 j jazz/"ens flzmmona a Jaw? MAI reef-zfl zfy.

Feb. 21, 1950 L. HAMMOND ETAL ELECTRICAL MUSICAL INSTRUMENT 8 Sheets-Sheet 7 Filed Sept. 15, 1944 EXPE/P/MENTAL CU/PI/E DE TERM/NED 5r EAR la s/ amne- 7'0 CHORUS EFFECT (Pu/e5 SINUSO/DAL TONE) GEA/ElQ/Y 7'50 cv/oleus FREQ UENC/ES 4 VA-Q14 c5 FEE 0 054 0) far 04 5 5 5a) i atented Feb. 21 1950 2.493367 ELECTRICAL MUSICAL: INSTRUMENT Laurens Hammond, Chicago, {and f Hanert,ParkRidge, Ill., aSSignoISjtGHamrniififl' Instrument company, Chicagri;*I1-l.,-a cameration of Delaware Application September 15, 1944, Serial No.554l80 Our invention relates generally to electrical musical instruments and more particul'arlyfto a f greatly improved form of chorus tone generating system for instruments of this character.

It is the fundamental object of our invention to provide an "improved electrical musical instrument in which there may be introduced a chorus effect which is of'much greater extent and tonal beauty than has previously been considered possible. -A. further object is to provide an electrical musical instrument having chorus generating equipment inv which the various electrical 'tone generators are eflecti-ve to supply groups of relativelypure tones havin average frequencies cor- ;respondingwto the notes of the musical scale and -in-which the percentage-of pitch deviation between the generators l of each group differs from ythatof the others ina manner to produce an improvedchorus efiect.

Another object .of our invention is to provide .chorus generating equipment in which a plurality of generators are employed for each of the-notes of the musical scale and in which the --corresponding chorus generators supplying the variousoctaves of the musical scale frequencies do not bear exact integer'relations-butaregenerated in a manner to deviate progressively from the standard octave. integer frequency relationship-to a diminishingextent with increasing frel quency.

=A further object of our invention is .to provide am improved'electrical musical instrument -in lcorporating chorusgenerators in addition to .solo

enerators, in which the various musical tone colorsa-re electrically synthesized, and in which the chorus generators supplying'the fundamental of a tone "differ from each other in pitch to -.a greater extent than the chorus generators supplyingthe higher harmonics.

It is another objectof our invention to provide chorus generating equipment in which the extent .of the chorus effect of each tone produced by a group of. simultaneously sounded: generators *lsachieved at a theoreticalmaximum,independ may of the particular tone quality employed.

It is another'object of our invention to provide chorus generating equipment in which the tuningof the chorus generators is not held to" a '3 constant percentage with respect to-the nominal frequencies of the notes of the musicalscale, but instead varies, depending upon the absolute J values 0'1. the nominal frequencies.

8 Claims. (Cl. 8 4 421) pairs "or chorus tone wheeigenerators taken on "the line 22" of 'Figf fi i :Fi' ureais asc ematic-circuit diagram showir'ig' a porno-nortrie'scloaswveu as the-chorus h generating. systemanu "illustrating mechanical fcircuit'siass'o'ciated'therewith;

"Figures 4,412 and db together constitute'aenart *settin-gifortn, in *t'abu-lated form var'ious physical com ants of the "g'enerating''systerrij permissible"percentage deviation or 'two'" "orus generators for different quality tones at different pitches;

Figure 6 is a curve comparing the chorus frequenci'es generated with the maximum permissible percentage deviation from the true pitch; and

Figure 7 is a chart setting forth, in tabulated form, various physical constants of a modified form of generating-system.

In order to appreciate fully theeffectproduced by the apparatus of our invention'it is necessary "to have'a clear-understanding of the frequency characteristics dot. chorus tone: gphenomena and z particularly of theabasicallyinew acoustic :principle -uponx which the JGhOIiLlS apparatus of our irrventi'oni'cperatea It-h l ong bee --r'ecognized that the-richness (it-ni'o'st" r'r i'usic2151 tonesimayrhegreatly lenhanced by the" utilizatien 4-HT several atone generating sources, all of which sound simultaneously :to produce a-cnoruseriectt Thisseffect is commonly observed in orchestras where it is not unusual to '--see a relatively laig e" 'numlier' of= musicians 5 playin "the samenote 'oiiithg rsame'lkind o'i instrunientt The eiiplanation of the resulting "beauty 'of 'th'eii colnpdsite tdnalitylresides in=1the fact that themusicians in'a'dvertantly' play -;at slightly d'iift-irei'tt pitches "The i esult' is an in- 40 teresting and pleasing 'toiial u'n'dul'ati'on;v .If all the musicianswere: 1ay at exactly the same -p'itch the resultin street wouldbe' substantially identical 6' that produced- 6y a single player save ror'ssemewriat-irrcreaseu Itiiidhss dueto the 45 multiplication er 1 the nuir'iber of instruments.

' These 'ihadvertant awareness in pitch, While pwductite of a very I i'o'tiriealile ch-ang'e' 'in quality are actually'-very --smal1' their mutual deviation ra'ctortein'g errata df-at'percent; Apparatus orus'efiects have also "been prOVicled-iiYciigansEaccordion rpsicnerus an-d't-helike. lienthe chdrus 'effect is-desired, ins'trumentsof thisclass mal'euse of several in dependent tone generators-- of slightly enter-m Other objects will appear from the following. .55: pitch rendered assua e-dust esents/e upen the description, references being had to the accompanyin drawings in. which:

Figure 1 isa fragmentary plan view of a-portionof thelchonus-generator Figure: 2'. is an enlarged sectional. view offtwo soft? and chnrus'es of voices;

. frequencies.

Builders of musical instruments have found that chorus effects are desirable only to the extent that the pitch difference between the several simultaneously sounded tone generators is relatively limited, in the order of .4%. If this percentage be increased, the effect tends to become musically unacceptable because the ear then resolves the overall chorus eifect into its individual tonal sources and perceives them as separate dissonant tones, and consequently evaluates the overall effect as an imperfection in tuning.

We have discovered that tone quality is of vital importance in determining the extent to which the pitches of the individual tone generators comprising the chorus may deviate. In general, the brighter tones of the strings and reeds may not deviate in pitch as much as the more mellow tones such as the organ diapason and fiute qualities. We have found that the maximum percentage deviation in pitch is permissible for pure sinusoidal tones. The following table illustrates the maximum extent of pitch deviation allowable when two independent sources of identical intensity and tone quality and the average pitch of which is 512 C. P. S., are sounded simultaneously:

The above figures strikingly show the interdependance of tone quality and extent of allowable pitch deviation between the chorus tone generators. These figures are based upon observations made by a trained musician. Similar observations by another musician might provide slightly different results, but. tests indicate that there is substantial agreement among musicians as to the permissible deviationvfor this chorus effect.

We have further discovered that the average pitch of the tone plays a major role in determining the maximum allowable percentage of pitch deviation. The curves of Fig. clearly shows this effect for the above qualities. Some of the interesting deductions which may be drawn from these curves follow:

(1) The maximum permissible percentage pitch deviation for very complex notes such as the orchestral strings and reeds is definitely limited in extent and rises but very little for the lower This may be understood by considering that the pitch of the string tone is extremely well defined because of its long harmonic series, all the frequencies of the tone being effective in a subjective manner to enable the ear accurately to ascribe a definite pitch to the sound. Thus a relatively small deviation in pitch of these bright tone generators results in the average listener being able to perceive the individual generators as separate dissonant tones.

(2) Of the five curves shown, the curve for the sinusoidal tone is especially interesting in that it shows by far the greatest permissible percentage pitch deviation between the chorus tone generators. This remarkable phenomenon is probably due to the factthat the sinusoidal tone is unique among all tone qualities in that it excites but a single sensory element in the listeners inner ear and thus its tonal pitch is perceived without the benefit of any subjective pitch reinforcement effects. Thus the sinusoidal tone represents, in a theoretical as well as a practical sense, the most difficult tone to which the ear may accurately assign a definite pitch. Consequently, the perceptive powers of the ear allow a very wide deviation in pitch between the two sinusoidal chorus generators before being able to resolve their separate pitches.

(3) The area under each of the curves is indicative, in a general way, of the extent of permissible chorus effect. It is at once apparent that the chorus effect may be greatest for the pure sinusoidal tone quality and should be least for the brilliant string tone quality. It is further seen that the ratio of these two extreme qualities is very large. However, the extensive chorus possibilities of the sinusoidal tone quality have been of little practical value because the sinusoidal quality of tone of itself is exceedingly uninteresting, being totally devoid of harmonic overtones.

From the preceding discussion it is apparent that the extent of permissible chorus effect is intimately related to, and limited by, the particular tone quality employed. In the past, this has imposed a fundamental limitation on the permissible extent of the chorus effect.

According to the principles of our invention, this interrelationship and consequent musical limitation is completely removed by separately generating the fundamental and the harmonic overtones, which make up the various musical tone qualities, in the form of individual groups of sinusoidal harmonic generators with each group having closely related frequencies but whose percentages of pitch deviation are appropriate to their particular average frequency, and which percentages vary in a substantial manner over the audio frequency range under consideration.

For example, the chorus generators supplying the fundamental of a string tone may have a relatively large percentage of frequency deviation, the second harmonic generators a smaller percentage deviation, the third and fourth harmonic generators a still smaller percentage deviation, etc. Thus, by the utilization of the principles of this invention, We are able to achieve a truly theroetical maximum chorus effect for all tone qualities. This follows by simply considering that each of the harmonics taken separately is sinusoidal in character, and thus each group of harmonic generators has percentage deviations conforming to the pure sinusoidal tone curve in Fig. 1. As point ed out heretofore, this curve for sinusoidal tones produces the theoretically greatest permissible chorus effect. Thus each of the partials of the harmonically synthesized chorus string tone, as generated by the apparatus of our invention, has approximately the same pitch deviation as is permissible with the sinusoidal tone. The resulting string tone is far more pleasing, by virtue of its extensive chorus effects, than a similar string tone in which the percentage of pitch deviation is relatively constant for the fundamental and harmonics alike. The latter, of course, is the case in which several complex wave generators sound simultaneously, as for instance several celeste" organ pipes.

The chorus effects of the usual complex musical tones, as synthesized by the apparatus of our invention, become unbelievably rich and full taking on a lovely liquid quality never before heard in all music. All traces of imperfections in s meness tuningas formerly perceived in the higher mar-a monivfrequencies-vanish; and-in their place. is';sub'stituted-a pleasingchorus of undulations; inithe lower order harmonics.- Heretofore,-- this la'tter'efiect has been impossible to achieve --with 5,- out objectionably' upsetting the pitch deviation of the -higher order harmonic chorus frequencies: This is" because I the percentage pitch deviation was-inherently the-same-for the fundamentalas: well as its harmonicsw The curves -of- Figs.=5 and-- 1o: 6 are-based upon a chorus generating system hav ing'two-generators for each pitch of the musical scale; When more than two generators-arepro vide'd foreachnote, somewhat greater pitch-dif ferences between the generators of the extreme pitches is permissible, apparently because I a greater 'portion of the'energy will necessarilybez ator near the nominal pitch;

The instrument of 'ourinventionpreferably takes the'form of'anelectrical organ'similar to; 29 th'at disclosed in thepatents to L. Hammond Nos-19 56,350 and 2,159,505. In thelatter patent there "is shown a set of solo generators and a setof chorus generators for the electrical synthesis of-various musical tone qualities; In the instrug5: merit disclosed in said patent the chorus generators provide fundamental and harmonic tonesdifieringfrom "the "corresponding 'sologenerators b'y'aconstant percentagein the order of .4%.

The "instrumentof 'our *inventionmay be con structed in the" manner disclosed-in the afore said Patent-1159505 except for the -fact that eachof -the tone wheelshaftswill carrytwo pair's or-chorus-tone wheels. The -swit'ching--mechanism and other general features ofconstruction of-the chorus generator andassociated partsmay be "of the construction shown in said patent:

A representative portion of a chorus generator: embodyingthe principles of the invention -is showirin Figs. 1 and'2 as comprising'fr'ame"plates 2 0 and 2! to which"separator'sheets' 22 are-se+ cured; The separator l sheets 22 I form shieldingcompartments 24 in eachzof whic'hare 'located two pairswofchorus generator tonewheelsdfii These tone' wheels 2 6; as shown in- Fig-2; are so cured to =hubs 28' which arenonrotatably se" cured to "a shaft" Sllmounted forrotation in suit able composition bearings 32 'secured'in the sheets 22?. A": gear 34 -is -mounted forrotation" on "th'e shaft snbetweem-the-two hubs: 285- A pair of 5 springs 36, 31 are wound about the hubs- 28 M: spect-ively' and are precompressed- "so-as :to press against' the opposite sides :of thegear-343 These springs thus serve as a :frictional and resilient. driving connection-between the gearr34 and-its shaft 30? As shown in Fig. 1, thedriven gears 34: meshwith"- drive pinions 38" nonrotatably :secured to an articulated main shaft 40;" The shaft 40.is driven through a -resilient driving connection-42 :fromaa motor drive shaft= 44 which: may constitute the armature shaft of a a synchronous or r constant speedsmotor:

Permanent 1 magnet rods 50' areradiustably1see cured in bushingsm secured to theiplate 21l, and their-Sinner ends, whichare general-1y wedgeshaped; are held inposition-lclosely adjacent the peripheries; of: the tone; or: aphonic: wheelsaxZG: Eachxofithe permanentimagnets fiihhassa pickup coilsx54z mounted thereon. adjacent its wedges. shaped end.

In Fig-e3 :there is. illustratedca (representative portion of theraschematiczwiringa diagram;; of 22a completes; orgam. there being disclosed? s'ixu solo genenatori tonenwh'eels 5.6;); 51;:583, 59a GDoa-ndrfii;

for generating:+frequendea -corresponding5-;to thez: fundamental,- second, 1 third, flfthw sixth rand? I eighth'm'armonic partials respectively ofr'a apars Y, ti'culartone; for example, the tone havingza lpitclrj;

CZ'J Eachvof 'these..: solo generators s-includes la pickup --magnet. 50 andi coil '54, The coils 54 are A connectedxinseries: with acondensers. C62 and the; primary winding of a-stransformer-..-'I63.,l to form:

a- -filter :circuithavinga resonant frequency cor-,2 responding a-to nthat 0f the F ,generaton The: sec-v ondaryswinding ofeach of the -transformers-.;Tfi3i=- has a pluralityrof: non-robbing or decoupling re-xr sisters 1 R64 connected thereto.

In Fi'g.-.3 .there is also illustrated a pluralitypf;

pairs of chorus generator tone wheels :66, 61; 68

and 69.= "'The pickup:coils-54 of these pairs of:

chorus generator i'tone wheels are adapted to :bel connected in: parallel with the pickup coils 54;. associated-with :thesolorgenerators- 58 to 6! respectively-=.'upon "closure 'ofswitches :12 which are preferablygang switches operable by means of a single manual control. A suitable gang switch of this type is shown in the aforesaid patent-to; HammandNo. 2,159,605.

A play-mg key C2,'1.through'an insulating 3.0-3. tuator:+l4, is adapted :to close :a plurality of key: switch'eslfi; Each of the switches comprises: asflexible arm l8 "connected-tonne -of "the re sistorsR64. vThe'other contacts of the switches 15 consist of bus bars all whichserveas collectors: for thersub-octave sub-fifth; fundamental, second; third, fourth, sixth", and-eighthharmonic: frequencies respectivelyw For illustrative purposes the bus bars are shownaslconnected to: bus s'bars 82 with 1 which 1 switch .arms 84 are adapted-to make contact upon depression oflapreah setskey 86. The bus barsr82 are;selectively.;cona. nectedsto.terminals- :88 ;.Which are connected to: suitably spaced" taps-on the primary Winding-"90: of a-transformer TM: The secondary winding.;94- of. ithiS transformer; is coupled @to output control: circuitss96 and the latter-zare coupled toranuame, plifierzfiathrough output controldevices l 0%); The amplifiereenergizes asspeaker; l02.'a The construe? tional details of; the apparatus v and.:circuits:may: bezsimilar {:to those. of .theaorgan .more 'fully de-f; scribed in' the aforesaidlPatentsNos.;1,956,350 and. 2155x505; i

It will be clear that whenever the chorus .effeo isldesired; the.switches sl2"are closed-,; thereby connectingseach" of the chorus tone wheel pickups in -rparallel with the solo tone .wheel pickups. Thus closure of oneof the key-operated switches 16 will causeatransmissionwto .the output of the organ of:=tone signals-including =notonly the ire-r quency-of thetsolo generator, which is that of the nominalvpitch of the note, but also two additional frequencies :generatedx by the chorus generators associated withtheselected solo generator.-; Referring; toathexzchartuof :Figs- 4, 4a and 412,-" it :will %be observed that: itconstitutes a: tabular. tionn-of the various1propertiessof the chorus generators: l

Column I isets forth 'L the :musi'cal designation of-nthemotmwhile column II represents; the num: ber of th GtSQlO generator (commencingxwith'the note Co as note No. 1), While colummllrindi's; cates the number 20f. :teethein the gchorus -genera- 13013;. drivingqgears; ompinionsa 38:;- Column; "IV simi'l'arly: :indicates the anumberz of. teeth in; the driven gearsy 34 associated with" the different chorus generators; Y

ColumnV-l shows thewnumbe'ri of. higheipoints onntheuph'onic wheels 26: .It .willbe. understood that l the; peripheries 'z-ofilthfisei tone .wheelseare 7 scalloped so as to generate substantially sine waves as they'rotate past the pickup magnets.

It will be noted that the first six pairs of chorus ilar manner, each series of six successive pairs,

of chorus tone wheel generators are similar. Because of this arrangement a substantial economy in themanufacture of the tone wheels is effected, since it is necessary to have only 16 different shapes of phonic wheels for producing the 96 different frequencies required in the chorus generator. It will also be noted that this selection of constants also permits-the duplication of driving and driven gears to a very substantial extent.

Column VI shows the actual frequencies which are generated by the chorus generators, while column VII shows the frequencies generated by the solo phonic wheels associated with the choru generators.

Column VIII sets forth the differences between the frequencies generated by the solo generators and those generated by their associated chorus generators. Column IX shows the ratio between the differences between the frequencies of column VIII and column VI, as a percentage of the fre quency of the solo generator.

Column X shows the, sum of the deviations of the chorus generators from the frequencies of the solo generators as av percentage of the solo generator frequencies. 1

From the frequencyidifierences set forth in column VIII, it will be clear that such deviation is generally in the order of 10 C. P. S. From column X it will be apparent that there is a substantial tapering of the deviation of the chorus generator frequencies from the frequencies of their corresponding solo generators, this percentage deviation varying from a maximum of 3.178% to a minimum of 563%, the greater deviation 1. e. in the order of more than 3% is utilized in the lower frequency spectrum, while the lesser frequency deviation is in the higherv pitch range in which it has been found tobe musically undesirable to have a wide frequency deviation.

1 By reference to the chart Figs. 4, 4a and 4b it will be clearly apparent that the deviations in frequency generated bythe chorus generators conform generally to the pure sinusodial tone curve of Fig. 5. The degree of correspondence is illustrated in Fig. 6 wherein the dash line curve corresponds generally tothe pure sinusoidal tone curve of Fig. and the irregular curve shown in full lines corresponds to the extent'ofdeviation of the chorus generator frequencies from the solo generator frequencies when the apparatus is constructed in accordance with the chart of Figs. 4, 4a and 4b. From Fig. 5 it will be clear that the frequencies generated by the chorus generators correspond generally with respect to their deviation from the solo generator frequency to the extent considered desirable by most musicians.

For some uses of the organ, for example, when used in churches, it is not desirable to utilize chorus generators having the maximum permissible deviation from the solo frequencies, and for such organs it is preferable that the chorus generators for the notes 44 to 61 deviate from their solo frequencies by a relatively constant percentage, in the order of .4%, making a total 9 chorus frequency range of .8% of the solo fre= quency. The frequency deviation of the chorus generators of such organs might follow the dot dash curve shown in Fig. 6 for the notes 44 to 61, but would preferably follow the full line curve for chorus generators above the note 61. An

organ constructed in this manner would constitute somewhat of a compromise between the instrument disclosed in said prior Patent 2,l59,- 505 and an organ incorporating in full the fea-,

tures of the present invention. Such compromise is desirable in organs used for ecclesiastical music because of the traditional preference for the simpler tone qualities in music of this character. v

i The chart, Fig. '7, sets forth the modified chorus generator details for the notes 44 to 61, it being understood that the chorus generators for the notes it to 9! would be constructed in the manner indicated in the tabulation of Figs. 4a and 4b. From Fig. 71 it will be noted that the total percentage range of deviation of the frequencies of.

the chorus generators, as compared with the solo generator frequency, is in the order of .8% (column IX, Fig. 7), while the differences in the frequencies of the two chorus generators of a pair is in the order of 1.575% (column X, Fig. '7).

It will be understood that for entertainment uses, the chorus effect is preferably used to its full extent, whereas for church use the full chorus effect in the lower pitches is generally considered undesirable because the tones are too lively and unusual for liturgical and similar purposes. Likewise, for use in the home, some musicians may prefer not to have the full range chorus effect extend to the lower notes, but instead, may prefer to.

have a relatively narrow chorus range for the notes G3 to F5.

Referring to Figs. 4, 4a and 4b, it will be noted that the chorus generators are divided into groups of semi-octave range by double horizontallines. It will further be noted that the numbers of teeth in the driving and driven gears for the first semi-octave correspond respectively to those of the second semi-octave (columns III and IV). By reference to column V (Fig. 4) it will be noted that the numbers of high points on the phonic wheels for the second semi-octave group differ from those in the first semi-octave group by a factor closely approximating the square root of 2, i. e. 1.4.14.

In a similar manner the third and fourth semioctave groups, the fifth and sixth semi-octave groups, and the seventh and eighth semi-octave groups respectively utilize driving and driven gears having the same numbers of teeth. The numbers of the high points on the phonic wheels for each of these semi-octave groups difier from the numbers of high points on the phonic wheels for the next adjacent semi-octave group by a factor closely approximating the square root of 2. Because of this relationship it is possible to mount two pairs of phonic wheels on each driven shaft and to reduce by one-half the number of driving and driven gears required'for the rotation of the chorus generators. It also makes it possible to use pairs of phonic wheels with the same numbers of high points for the chorus generators of each semi-octave group. The manufacture of the chorus generator apparatus is thus greatly simplified.

From the foregoing it will be clear that we have devised a chorus generating apparatus capable of producing musical chorus effects inherently unobtainabie by any prior musical instrument in that the degree of chorus effect varies throughout the musical scale in a general accordance with the response characteristics of the human ear. Thus each partial of each musical tone produced will have the most desirable degree of chorus frequencies added thereto. The overall effect of music produced by the instrument is thus of a vitally different character and a substantial improvement as compared with the music which may be produced by any other instrument.

While we have shown and described particular embodiments of our invention, it will be clear to those skilled in the art that quite a number of additional variations and modifications of the invention may be made without departing from the underlying principles thereof. We therefore desire, by th following claims, to include within the scope of our invention all such modifications and variations by which substantially the results of the invention may be obtained by the use of substantially the same or equivalent means.

We claim:

1. In an electrical musical instrument having electrical sine wave solo generators respectively generating frequencies of the notes of the tempered musical scale extending at least from the frequency 392 to 5924.57 cycles per second, and a plurality of electrical chorus generators, and means to cause said generators to generate frequencies respectively differing from those of the solo generators by different percentages, those in the lowest frequency range differing by a percentage in the order of 1.6, those in the higher frequency range differing by a percentage in the order of .28 and those in the ranges intermediate these extreme frequencies differing by percentages between 1.6 and .28.

2. In an electrical musical instrument having a set of solo generators generating sine waves of substantially the frequencies of the tempered musical scale, a plurality of chorus generators, and means to cause said generators to generate frequencies differing respectively from the frequencies generated by said solo generators, the frequency differences between the chorus generators and their respective solo generators being relatively small for the higher pitch notes and increasing progressively in descending the musical scale.

3. An electrical musical instrument having a plurality of electrical generators, there being a set of generators for each note of the musical scale within a predetermined gamut and means to operate, the generators of each set to cause them to generate frequencies differing from one another but having an average frequency substantially that of a note of the tempered musical scale, the maximum percentage deviation from the average of the frequencies generated by the generators of each set differing throughout the musical scale, there being a greater deviation at the lower end of the scale and a lesser deviation at the upper end of the scale.

4. In a chorus generator, the combination of a plurality of tone wheel shafts, two pairs of generator tone wheels mounted in spaced relation on each of said shafts, means for driving said shafts at constant speeds, the average number of high points on one pair of tone wheels on each shaft differing from the average number of high points on the other pair of tone wheels on such shaft by a factor in the order of the square root of 2.

5. In an electrical musical instrument, apparatus for producing a chorus effect comprising, a plurality of constant frequency solo generators of relatively fixed pitch whose frequencies are those of the equal tempered scale, a plurality of chorus generators respectively associated with said solo generators, and means to cause the pitches of the chorus generators to be maintained accurately relative to the solo generators at such pitches that the chorus generators at the high frequency end of the scale generate frequencies which differ percentagewise from their respectively associated solo generators by a smaller factor than do the chorus generators at the lower frequency end of the scale.

6. In an electrical musical instrument, having a series of solo generators of electrical impulses at the frequencies of the notes of the tempered musical scale, a plurality of pairs of electrical chorus generators respectively connectable in parallel with said solo generators, means to cause said chorus generators to produce electrical impulses of frequencies differing from the frequencies of the solo generators by factors in the order of .0055 in the range about 1760 cycles per second, .0042 in the 2400 cycles per second range, .0040 in the 3500 cycles per second range, and .0028 in the 5000 cycles per second range, and means operable at will for electrically connecting said pairs of chorus generators respectively in parallel with said solo generators.

'7. In an electrical musical instrument, having a series of solo generators of substantially sine wave electrical impulses at the frequencies of the notes of the tempered musical scale from approximately 392 cycles per second to approximately 5925 cycles per second, a plurality of pairs of chorus generators respectively connectable in parallel with said solo generators, means to cause said chorus generators to produce substantially sine wave electrical impulses of frequencies differing from the frequencies of the solo generators by about 6 to 8 cycles per second in lowest octave of the chorus generator gamut, by about 8 to 11 cycles per second in the next to the lowest octave, by about 9 to 13 cycles per second in the third lowest octave, and by about 11 to 24 cycles per second in the highest octave, and means for selectively connecting the outputs of said pairs of chorus generators respectively in parallel with the outputs of said solo generators.

8. An electrical musical instrument in which complex musical tones are produced by combining a plurality of harmonically related substantially pure sine waves, comprising, a plurality of solo electrical generators of substantially pure sine waves of the frequencies of the notes of the musical scale, a plurality of chorus electrical generators of substantially pure sine waves of frequencies differing from those generated by the solo generators by small percentages which decrease with increases in the frequencies generated, and selectively operable switch means for combining the electrical output of each chorus generator with the electrical output of the solo generator of the nearest frequency.

LAURENS HAMMOND. JOHN M. HANERT.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Hammond May 23, 1939 

